Natural frequency is a characteristic of every machine, structure, and object. The natural frequency is the frequency of free vibration of a system, in which the system vibrates to dissipate its energy. The angular natural frequency (wn) of an object, expressed in radian per second, is a function of its stiffness (k) and its mass (m), as exhibited by the following equation:
      w    n    ⁢            k      m      If either the stiffness (k) or the mass (m) of the object is altered, the natural frequency will change.
When a natural frequency is excited by some external force, an increase in the amplitude of the vibration may result. Such external force may be created by driving the rotation of a drive line. When the natural frequency of the drive line is excited, the operating speed of the system is defined as the “critical speed”. If the drive line operates at the critical speed and dampening of the drive line is not present, the vibration created by the natural frequency could damage or destroy the drive line.
The vibration created by such natural frequencies in a drive line can have negative consequences when applied to various test equipment for rotating devices. For instance, in today's automotive industry, engines and transmissions must be cold and hot-tested after they are assembled on the factory floor but before they are assembled into a motor vehicle. Such tests often involve connecting the engine or transmission to a drive line system that rotates or spins the engine or transmission while being coupled to a test machine. In testing the engine or transmission, the drive line system rotates or spins the engine or transmission at various operating speeds or revolutions per minute (rpms) in order to test the engine and transmission under various conditions. However, when driving or rotating the shaft of the test machine, the shaft may reach a natural frequency, thereby causing the drive line system to resonate and vibrate. This will, of course, affect the test results of the engine or transmission, thereby providing possible false indicators as to the test performance of the engine or transmission. In worse case scenarios, the resulting resonance and vibration from a natural frequency occurrence could affect or damage the engine or transmission. Although changing the rotational speed of the drive line could avoid realizing the natural frequency of the drive line, such test equipment typically requires that the tested products rotate at specific rates and times thereby prohibiting the option of changing the drive line speeds.
It would be desirable to provide a coupling apparatus that could effectively change the natural frequency of a drive line so as to avoid the vibration and resonance associated with a particular natural frequency of the drive line.